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// Distributed under the Boost Software License, Version 1.0. (See accompanying

// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

// This program demonstrates the use of leaf::on_error to capture the path an

// error takes as is bubbles up the call stack. The error path-capturing code

// only runs if:

// - An error occurs, and

// - A handler that takes e_error_trace argument is present. Otherwise none of

// the error trace machinery will be invoked by LEAF.

// This example is similar to error_log, except the path the error takes is

// recorded in a std::deque, rather than just printed in-place.

#include <boost/leaf.hpp>

#include <iostream>

#include <deque>

#include <cstdlib>

#define ENABLE_ERROR_TRACE 1

namespace leaf = boost::leaf;

// The error trace is activated only if an error handling scope provides a

// handler for e_error_trace.

struct e_error_trace

{

struct rec

{

char const * file;

int line;

friend std::ostream & operator<<( std::ostream & os, rec const & x )

{

return os << x.file << '(' << x.line << ')' << std::endl;

}

};

std::deque<rec> value;

friend std::ostream & operator<<( std::ostream & os, e_error_trace const & tr )

{

for( auto & i : tr.value )

os << i;

return os;

}

};

// The ERROR_TRACE macro is designed for use in functions that detect or forward

// errors up the call stack. If an error occurs, and if an error handling scope

// provides a handler for e_error_trace, the supplied lambda is executed as the

// error bubbles up.

#define ERROR_TRACE auto _trace = leaf::on_error( []( e_error_trace & tr ) { tr.value.emplace_front(e_error_trace::rec{__FILE__, __LINE__}); } )

// Each function in the sequence below calls the previous function, and each

// function has failure_percent chance of failing. If a failure occurs, the

// ERROR_TRACE macro will cause the path the error takes to be captured in an

// e_error_trace.

int const failure_percent = 25;

leaf::result<void> f1()

{

ERROR_TRACE;

if( (std::rand()%100) > failure_percent )

return { };

else

return leaf::new_error();

}

leaf::result<void> f2()

{

ERROR_TRACE;

if( (std::rand()%100) > failure_percent )

return f1();

else

return leaf::new_error();

}

leaf::result<void> f3()

{

ERROR_TRACE;

if( (std::rand()%100) > failure_percent )

return f2();

else

return leaf::new_error();

}

leaf::result<void> f4()

{

ERROR_TRACE;

if( (std::rand()%100) > failure_percent )

return f3();

else

return leaf::new_error();

}

leaf::result<void> f5()

{

ERROR_TRACE;

if( (std::rand()%100) > failure_percent )

return f4();

else

return leaf::new_error();

}

int main()

{

for( int i=0; i!=10; ++i )

leaf::try_handle_all(

[&]() -> leaf::result<void>

{

std::cout << "Run # " << i << ": ";

BOOST_LEAF_CHECK(f5());

std::cout << "Success!" << std::endl;

return { };

#if ENABLE_ERROR_TRACE // This single #if enables or disables the capturing of the error trace.

[]( e_error_trace const & tr )

{

std::cerr << "Error! Trace:" << std::endl << tr;

#endif

[]

{

std::cerr << "Error!" << std::endl;

} );

return 0;

}

////////////////////////////////////////

#ifdef BOOST_LEAF_NO_EXCEPTIONS

namespace boost

{

[[noreturn]] void throw_exception( std::exception const & e )

{

std::cerr << "Terminating due to a C++ exception under BOOST_LEAF_NO_EXCEPTIONS: " << e.what();

std::terminate();

}

struct source_location;

[[noreturn]] void throw_exception( std::exception const & e, boost::source_location const & )

{

throw_exception(e);

}

#endif

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